Application of nitride semiconductors having a high saturation electron velocity and a wide band gap to high-withstand-voltage, high-power semiconductor devices is being considered. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV that is greater than the band gap 1.1 eV of Si and the band gap 1.4 eV of GaAs, and has a high breakdown field strength. For this reason, a nitride semiconductor such as GaN is a very promising material for a high-voltage-operation, high-power semiconductor device for a power supply.
Many reports have been made on field effect transistors, particularly, high electron mobility transistors (HEMT), which are examples of semiconductor devices using nitride semiconductors. As an example of a HEMT using nitride semiconductors, an InAlN/GaN HEMT, which uses GaN as a channel layer and InAlN as a barrier layer, is getting attention. In an InAlN/GaN HEMT, the lattice matching between InAlN and GaN can be achieved and a high-quality crystal film can be obtained by setting the composition ratio of In at 17% to 18%. Also, when InAlN is formed with such a composition ratio, the formed InAlN has very high spontaneous polarization. Therefore, an InAlN/GaN HEMT can generate a two-dimensional electron gas (2DEG) having a density greater than the density of the two-dimensional electron gas generated by an AlGaN/GaN HEMT using AlGaN for a channel layer. For the above reason, HEMTs using InAlN as the barrier layer are getting attention as next-generation high-power devices (see, for example, Japanese Laid-Open Patent Publication No. 2010-74047; and F. A. Faria et al., “Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy,” Appl. Phys. Lett., 101, (2012) 032109).